Localization of diuretic effects along the loop of Henle: an in vivo microperfusion study in rats

Abstract

In order to clarify the effects on sodium reabsorption in the loop of Henle of methazolamide (a carbonic anhydrase inhibitor), chlorothiazide and the loop diuretics frusemide and bumetanide, superficial loops were perfused in vivo in anaesthetized rats and the individual diuretics were included in the perfusate. Differentiation between effects in the pars recta and in the thick ascending limb of Henle (TALH) was achieved by comparing responses to the diuretics when using a standard perfusate, designed to mimic native late proximal tubular fluid, and a low-sodium perfusate, designed to block net sodium reabsorption in the pars recta. With the standard perfusate, methazolamide caused decreases in sodium reabsorption (J(Na)) and water reabsorption (J(V)); with the low-sodium perfusate, a modest effect on J(Na) persisted, suggesting that carbonic anhydrase inhibition reduces sodium reabsorption in both the pars recta and the TALH. The effects of chlorothiazide were very similar to those of methazolamide with both the standard and low-sodium perfusates, suggesting that chlorothiazide also inhibits sodium reabsorption in the pars recta and TALH, perhaps through inhibition of carbonic anhydrase. With the standard perfusate, both frusemide and bumetanide produced the expected large decreases in J(Na), but J(V) was also lowered. With the low-sodium perfusate, the inhibitory effects of the loop diuretics, particularly those of frusemide, were substantially reduced, while net potassium secretion was found. These observations indicate that a significant component of the effect of frusemide (and possibly of bumetanide) on overall sodium reabsorption is located in the pars recta, and that loop diuretics induce potassium secretion in the TALH.

Effects of the Potassium Channel Blocker Barium on Sodium and Potassium Transport in the Rat Loop of Henle In Vivo

“…The data confirm the previous observation (Unwin et al 2000) of net potassium secretion (manifest as a negative value for J K ) when bumetanide was included in the low-Na + perfusate. The addition of barium reduced the magnitude of this potassium secretion; however, J K remained significantly different from zero.…”

“…A second objective of the present investigation was to assess the validity of an in vivo model of luminal K + channel activity in the TALH. We have shown previously that the addition of a loop diuretic such as bumetanide to the low-Na + perfusate not only virtually abolishes the remaining sodium reabsorption in the loop but also reverses the direction of net K + transport, resulting in significant K + secretion into the lumen (Unwin et al 2000). Our hypothesis is that this secretion is a consequence of potassium movement out of the cells through the luminal K + channels in the TALH.…”

Net K + secretion in the thick ascending limb of mice on a low-Na, high-K diet

“…Along with the opposite effect in mice on CTRL, we provide strong evidence for net K + secretion in the TAL. Additionally, furosemide may inhibit carbonic anhydrase (CA) 36 , causing more K + following Na + delivery to the EDT, which would increase the EDT [K + ]. Therefore, if furosemide inhibits CA, the actual amount of furosemide-sensitive K + secretion in the TAL of LNaHK mice would be greater than that observed.…”

Analysis of energy metabolism and mechanism of loop diuretics in the thick ascending limb of Henle's loop in dog kidneys

“…1998) excludes an effect of loop diuretics on more distal nephron segments. In an in vivo study on rats Unwin et al . (2000) perfused pars recta of the proximal tubules and collected the perfusate in the first accessible segment of the distal tubules.…”

“…Moreover, the confinement of Na-K)2Cl transporter to TALH (Nielsen et al 1998) excludes an effect of loop diuretics on more distal nephron segments. In an in vivo study on rats Unwin et al (2000) perfused pars recta of the proximal tubules and collected the perfusate in the first accessible segment of the distal tubules. Addition of furosemide or BUM to the perfusate reversed the direction of potassium transport from absorption to secretion.…”